The notion of Level (in a scale of Levels) is probably the most authentic notion in Architecture. Already in the work of Vitruvius the notion is implicitly present in the triad ‘ordinatio – symmetria – eurythmia’. In more recent times, the notion always appears in relation with hierarchical organization as a means of control of quality. However used in drawings and in architectural discourse, the term lacks precision; there are many types of level like abstraction, specification, dependency, resolution levels etc., but no operational definition can be found as a notion that structures architectural objects and design processes simultaneously in a consistent way. Defining this notion of Level is the purpose of this paper. An example of application in an architectural decision-making process completes the paper.